Make sure that your seat really is right.
Put slicks on the bike to improve road handling.
Don't be scared to give yourself rest days, especially at first.
If pain returns, have your bike fit checked. A common error is to have the saddle too low. If your store won't help then post pictures of you riding the bike here.
Food: follow the standard diet advice that everyone should. Do NOT drink non-diet sodas - they're loaded with fructose, which will mess up your metabolism. Eat as little processed food, especially that containing HCFS, as you can. Avoid fruit juice too, but eat fruit (the roughage in unjuiced fruit makes the fructose safe.) Eat wholemeal pasta and bread, baked and boiled potatoes, chicken, lean red meat. Eat oily fish at least twice a week.